Alzheimer's disease is a disease characterized by degeneration and loss of neurons as well as formation of senile plaques and neurofibrillary degeneration. Currently, Alzheimer's disease is treated only with symptomatic treatment using a symptom improving agent typified by an acetylcholinesterase inhibitor, and a fundamental remedy to inhibit progression of the disease has not yet been developed. It is necessary to develop a method for controlling the cause of the onset of pathology in order to create a fundamental remedy for Alzheimer's disease.
It is thought that Aβ-proteins as break-down products of amyloid precursor proteins (APP) are involved in the degeneration and loss of neurons and onset of symptoms of dementia (see, e.g., Klein et al., Alzheimer's disease-affected brain: Presence of oligomeric Aβ ligands (ADDLs) suggests a molecular basis for reversible memory loss, PNAS USA 2003, Sep. 2; 100 (18), p. 10417-10422; Nitsch et al., Antibodies against β-amyloid slow cognitive decline in Alzheimer's disease, Neuron, 2003, May 22; 38, p. 547-554).
Aβ-proteins have, as main components, Aβ40 consisting of 40 amino acids and Aβ42 with two amino acids added at the C-terminal. The Aβ40 and Aβ42 are known to have high aggregability and to be main components of senile plaques (see, e.g., Jarrett et al., The carboxy terminus of the β amyloid protein is critical for the seeding of amyloid formation: Implications for the pathogenesis of Alzheimers' disease, Biochemistry, 1993, 32 (18), p. 4693-4697; Glenner et al., Alzheimer's disease: initial report of the purification and characterization of a novel cerebrovascular amyloid protein, Biochemical and biophysical research communications, 1984, May 16, 120 (3), p. 885-890; Masters et al., Amyloid plaque core protein in Alzheimer disease and Down syndrome, PNAS USA, 1985, June, 82 (12), p. 4245-4249). Further, it is known that the Aβ40 and Aβ42 are increased by mutation in APP and presenilin genes which is observed in familial Alzheimer's disease (see, e.g., Gouras et al., Intraneuronal Aβ42 accumulation in human brain, American Journal of Pathology, 2000, January, 156 (1), p. 15-20; Scheuner et al., Secreted amyloid β-protein similar to that in the senile plaques of Alzheimer's disease is increased in vivo by the presenilin 1 and 2 and APP mutations linked to familial Alzheimer's disease, Nature Medicine, 1996, August, 2 (8), p. 864-870; Forman et al., Differential effects of the swedish mutant amyloid precursor protein on β-amyloid accumulation and secretion in neurons and nonneuronal cells, The Journal of Biological Chemistry, 1997, Dec. 19, 272 (51), p. 32247-32253). Accordingly, a compound that reduces production of Aβ40 and Aβ42 is expected to be a progression inhibitor or prophylactic agent for Alzheimer's disease.
Aβ is produced by cleaving APP by beta-secretase (BACE1) and subsequently by gamma-secretase. For this reason, attempts have been made to create gamma-secretase and beta-secretase inhibitors in order to inhibit Aβ production.